552 



EXPLORATION GEOPHYSICS 



transient in the current circuit and (b) the potential transient in 

 the circuit which includes the potential electrodes. The chief purpose 

 of the investigations was to determine whether the transients contained 

 ripples which were associated with anomalous subsurface structural 

 conditions. 



By using a camera of the high speed revolving drum type to 

 photograph the spot of light produced by a cathode-ray oscillograph 

 tube, Hawley showed that the current build-up time in the energizing 

 circuit varied from 30 microseconds for a 12,000 foot spread to 4.5 

 microseconds for a 3,000 foot spread. The voltage transient surge 

 showed an almost instantaneous rise to its maximum value, followed 

 by an exponential decrease. The voltage and current transients are 

 typical of inductive, highly damped circuits. The work did not show 

 the presence of any ripples on any of the curves, even with recording 

 speeds sufficiently great to reveal ripples having a duration of only 

 10 microseconds. 



In the present utilization of the transient methods, the original 

 concept of the presence of ripples and their diagnostic value apparently 

 has been abandoned, and interpretation is based on fundamental resistivity 

 relationships. The spread of current through a bed of high resistivity 

 is much faster than that through a bed of low resistivity. As a result, 

 the current transient in a medium of high resistivity is steeper (less time 

 interval) than that in a medium of low resistivity. The time constant 



and the shape of the wave there- 

 fore furnish a means for deter- 

 mining the relative resistivities of 

 the materials included within the 

 effective path of current flow. Two 

 typical transient curves are shown 

 in Figure 339,t The electrode and 

 apparatus arrangement are essen- 

 tially the same as that illustrated 

 in Figure 338. 



As previously mentioned, the 

 photographic or oscillographic 

 method for measuring the wave 

 form of reoccuring phenomena has 

 an advantage in eliminating the 

 effects of extraneous disturbances. For this work Klipsch % stops down 

 the recording camera lens until each single sweep of the oscillograph ray 

 gives a faint image. The wave is repeated a sufficient number of times 



(a) (b) 



Fig. 339. — -Typical transient curves for (a) 

 material^ of high resistivity and (b) material of 

 low resistivity. (After Steinmann, The Oil and 

 Gas Journal.) 



t K. W. Steinmann, "Use of the Transient and Soil Analysis Methods in the Search for Oil," 

 The Oil and Gas Journal, July 27, 1939, pp. 85-87. 



t P. W. Klipsch, "Recent Developments in Eltran Prospecting," Geophysics, Oct. 1939, Vol. IV, 

 No. 4, pp. 283-291. 



